Bioluminescent countershading in midwater animals: evidence from living squid

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Science  12 Mar 1976:
Vol. 191, Issue 4231, pp. 1046-1048
DOI: 10.1126/science.1251214


Midwater squid respond to overhead illumination by turning on numerous downward-directed photophores; they turn off the photophores when overhead illumination is eliminated. The squid are invisible when the intensity of the photophores matches the intensity of the overhead illumination. These results strongly support the theory of ventral bioluminescent countershading.